Auger electron spectroscopy

ABSTRACT

An apparatus is disclosed having special utility for performing analyses utilizing the Auger effect. A coaxial cylindrical electron analyzer is combined with a coaxial electron gun between the apertures in the inner tube of the analyzer. A structure for a cylindrical analyzer having an electrical field free of fringing between cylinders is also disclosed, as is a simplified off-axis exit aperture.

United States Patent 1 Bohn et al.

[ June 12, 1973 AUGER ELECTRON SPECTROSCOPY 3,553,451 1/1971 Uthe 250/419 DS [75] Inventors: Gerald K. Bohn, Hopkins; OTHER PUBLICATIONS g g i i' w Mmneapohs High Sensitivity Auger Electron Spectrometer Palmot 0 berg, Applied Physics Letters, Vol. 15, No. 8 Oct. 1969 [73] Assignee: Physical Electronics Industries, Inc.,

Edina, Minn. Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church [22] Filed 1971 Attorney-Schroeder, Siegfried & Ryan [21] Appl. No.: 210,092

Related U.S. Application Data ABSTRACT [63] Continuation of Ser. No. 68,983, Sept. 2, 1970, An apparatus is disclosed having special utility for perabandoned. forming analyses utilizing the Auger effect. A coaxial cylindrical electron analyzer is combined with a coaxial [52] U.S. Cl. 250/495 AE electron gun between the apertures in the inner tube of [51] Int. Cl. H01 j 37/26 the analyzer. A structure for a cylindrical analyzer hav- [58] Field of Search 250/495 AE, 49.5 PE ing an electrical field free of fringing between cylinders is also disclosed, as is a simplified off-axis exit aperture. [56] References Cited 1 C 4 D F UNITED STATES PATENTS 3,609,352 9/1971 Harris 250/495 AE RECORDER J 1- g LOCK- IN AMPLIFIER 16 IJ5K- ELECTRON GUN 1: l I i \1 7 I l 1 a I l l r l 1 AUGER ELECTRON SPECTROSCOPY This application is a continuation of Ser. No. 68,983, filed Sept. 2, 1970, now abandoned.

The present invention is directed to an apparatus for performing analyses which makes use of electrons which are emitted from a substance after being bombarded with electrons from a source such as an electron gun. The technique to which the invention is specifically directed is to what is known as Auger spectroscopy. In this type of a technique, a target material is placed into a vacuum, usually below Torr. Upon being bombarded by electrons from some source, such as an electron gun, the sample under consideration gives off a variety of emissions. Among these are X- rays, secondary electrons and reflected primary electrons from the source. The target material also gives off Auger electrons in the manner which is well publicised in the literature.

In prior art apparatus making use of the Auger effect, a variety of electron analyzers have been utilized. For example, the so-called sector analyzer has found use, as has LEED (low energy electron defraction apparatus).

In apparatus which utilizes a electron analyzers of the sector analyzer type, the impinging of the electrons from the electron gun on the sample being analyzed has been at what is termed a grazing incidence. Grazing incidence has heretofore also been used for the coaxial cylinder type of electron analyzer. An example of this latter type of apparatus is described in an article entitled High Sensitivity Auger Electron Spectrometer" appearing in Applied Physics Letters, Vol. 15, No. 8, for Oct. 15, 1969, beginning at page 254.

Certain sensitivity enhancement is possible through the use of grazing incidence. However, a number of disadvantages are also present in the use of grazing incidence. Among these disadvantages are the relatively large amount of space consumed by the grazing incidence construction. This is of particular importance when it is appreciated that the technique of Auger spectroscopy requires very high vacuums and any additional space consumed by equipment merely adds to the difficulty of the user in making fullest use of the procedure. A second and more serious disadvantage of the grazing incidence construction is the problem of obtaining alignment between sample, electron gun, and analyzer. This latter problem is closely tied to a third problem of the prior art, which is the necessity of using separate mounting flanges for the analyzer and the electron gun.

THE INVENTION In accordance with our invention, we have provided an apparatus for analytical purposes which combines the full potential of the cylindrical analyzer, along with the advantages of having a highly compact apparatus which is easily maintained in alignment with the samples being analyzed. Apparatus in accordance with our invention is constructed and arranged so that the source of primary electrons lies intermediate the annular openings of the inner cylinder of the cylindrical analyzer. The apparatus is so oriented that the emitted beam of electrons generated by the electron gun strikes the sample under test. The Auger electrons which result are passed through the analyzer and around the electron gun to the detector.

A further improvement in accordance with our invention is to the use of a ceramic spacer at or near the ends of the cylindrical tubes which is coated so as to be conductive, thus resulting in a near perfect termination of electric fields between the inner and outer cylinder.

A still further improvement in accordance with our invention is an off-axis exit aperture which gives greatly improved resolution of the analyzer.

Our invention will be best understood from a consideration of the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of the invention, including the circuitry and magnetic shielding;

FIG. 2 is an elevational view in section of the analyzer and electron gun construction of the preferred form of the invention;

FIG. 3 is a sectional view along lines 3-3 of FIG. 2; and

FIG. 4 is an end elevational view of the exit aperture of FIG. 2.

Turning first to FIG. 1, there is schematically illustrated an apparatus in accordance with the present invention. The cylindrical analyzer is composed of two members 11 and 12. These members consist of concentric cylindrical tubing. Member 12 has openings 13 and 14 near opposite ends thereof in a generally annular configuration, as will be explained below. These openings will generally have a fine mesh screen thereacross which permits the electrons to travel therethrough, but aids in maintaining a uniform electric field between members 11 and 12. An electron gun is positioned within the bore of tube 12 and spaced between openings l3 and 14. Typical dimensions will be approximately one inch OD for tube 12 and 3 inches for tube 11. Overall length of the tubes is about three inches.

A target member 15 of the sample to be analyzed is positioned at the open end of tube 12 and in close proximity thereto. Various mounting structures known in the vacuum art can be used to hold and position the target 15. Electrons emitted from the electron gun generally indicated by dotted line 16 are directed to the surface of target 15. Auger electrons, among other emissions, are emitted from the surface of the target. Some of these electrons which are emitted pass through opening 13. These are generally indicated by the two curved lines extending from sample 15. By application of the negative potential to tube 11, the electrons are deflected from their initial straight line path and follow a generally curved path, as shown, between opening 13 and 14 and are refocused at the opposite end of tube 12. By application of a negative potential to tube 11, those electrons having a specific velocity will be focused onto the electron multiplier tube 17 and will be detected at that point. Electrons which have higher or lower velocity than some predetermined value will not be focused to the detector 17, but will be collected at some other point along their path.

For sake of clarity, the vacuum chamber necessary for operation of the invention is schematically shown by dashed line 20. Magnetic shielding is schematically illustrated as 20a. Such shielding is desirable when greatest precision is sought, as any extraneous influence on the electrons in the analyzer detract from analysis.

By application of suitable direct current potential, by means of the variable source 18, one can scan an entire range of electron energies in the manner which is known in the art. By superimposing a time variable voltage on the main voltage, one can differentiate the signal to provide a greater degree of definition of Auger electrons from other constant source electrons that are collected. These matters are known in the art. They are described, for example, in the article referred to above appearing in the Applied Physics Letters.

Also illustrated in FIG. 1 is a second electron gun schematically shown as 19. An electron gun in such a position can be utilized to produce a grazing incidence operation of the type shown in the prior art. As can be readily seen, the coaxial electron gun of the present invention does not interfere with such use of the grazing incidence gun of the prior art. This illustrates the additional flexibility of an apparatus in accordance with the present invention.

Referring now to FIG. 2, there is shown in side elevational view and in section the construction of the concentric tube portion of the analyzer and an electron source in accordance with our invention. The concentric tube analyzer consists of two tubular members 21 and 22, which are held in spaced relationship to one another by a ceramic insulator material 23. Annular ring 23 is most desirably of a material such as alumina or quartz. Members 21 and 22 are advantageously made of a stainless steel. Unless specified to the contrary, we have found that stainless steel is desirable for all metallic portions of the apparatus.

Inner tube 22 is provided with annular openings 24 and 25 at a point near each end of tube 22. As best seen in FIG. 3, the annular openings extend completely around tube 22, with the exception of support members 26, which connect the end portions of tube 22 to the main central body. Across the openings provided in tube 22 there is provided a stainless steel mesh of approximately 100 lines per inch, which is 80 percent transparent. This is illustrated by dotted line 42. By the use of the stainless steel mesh across the openings, one minimizes electric field distortions between the inner and outer cylinders.

As a further aid to maintaining an electrical field between outer cylinder 20 and inner cylinder 21 without the problem of field fringing, l have found that a thin, high resistance cermet material can be coated on the inner surface of members 23 so as to provide an electrical path, but one of very high resistance. I have found that a resistance of approximately 30 megohms accomplishes the desired purpose, although a resistance of from l-l00 megohms is suitable. This can be accomplished in a variety of ways. One can deposit thin films of germanium or silicon, but these materials have relatively high temperature coefficients of resistivity. The preferred form is to form a cermet coating from aluminum oxide (alumina) or of silicon dioxide (quartz) and nickel metal by vacuum sputtering these materials onto the inner surface of ring 23 to form a thin layer having the desired resistance. The outer and inner rims of member 23 are coated with a metal film to provide electrical contact to the cylinders and to the resistive coating. The result of this construction is a near perfect termination of electric fields at the ends of the chamber without the high temperature change effects of the prior art.

Interior to tube 21 and'located between openings 24 and 25 is the source of electrons such as an electron gun. The electron gun is of a construction known in the art and consists of a mounting member 27 at one end thereof to provide support and to maintain the gun in alignment along the axis of tube 22. Member 27 will be provided with openings to permit access of the necessary electrical leads to the anodes and cathode of the gun. Spacer bars 28 of an insulating material, such as alumina, are provided for mounting the cathode 29, which is illustrated as being a pair of filament holding members. A plurality of anode members 30, 31 and 32 are provided which, in combination with columator 33, act to focus the electrons into a fine beam. Ceramic spacer members 34 and 35 serve to electrically isolate and support the anode members. As can be seen, anode 32 is in contact with tube member 22. Its potentialis therefore the same as tube 22. Columator 33 and anode 31 are likewise of the same potential.

The means for providing electrical contact to the electron gun cathode and anodes is by way ofa tube 36 which is shown extending into the tube 22 from the right-hand portion of FIG. 2. Tube 36 is desirably of a metal of high magnetic permeability to provide magnetic shielding and which desirably has an interior ceramic coating to provide additional electrical insulation. For sake of clarity of drawings, the leads themselves are not shown. It is desirable to locate tube 36 in such a position as to be shielded from interfering unnecessarily with passage of electrons through opening 25 by placing tube 36 adjacent rib member 26. For the greatest sensitivity of the overall system, it is desirable to provide magnetic shielding of all electrical leads contained within tube 22, thereby avoiding interference with the path of the Auger electrons through the cavity defined between tubes 21 and 22. Tube 36 provides such shielding for leads. As illustrated, tubes 21 and 22 terminate with insulator 23. However, they may extend beyond, although some additional problems arise in positioning a sample to be analyzed.

Operation of the electron gun described with regard to FIG. 2 is conventional and will not be described with any particularity herein. As has been noted, it is important for greatest sensitivity that the electron gun be magnetically shielded wherever possible.

It has been recognized by prior investigators that the cylindrical mirror analyzer formed by tubes 21 and 22 can have its resolution improved if one positions an exit slit off of the axis of the tubes. This is discussed in an article by I-Iafner et al. entitled Comparison of the Spherical Deflector and the Cylindrical Mirror Analyzers, appearing in the Review ofScientific Instruments, Vol. 39, No. l, for Jan. l968, beginning at page 33. In the article appearing in the Applied Physics Letters referred to above, an arrangement was provided for achieving the improved resolution. In accordance with this latter articles teachings, resolution was improved by providing a rod member in conjunction with a centrally located opening in the exit aperture.

Prior art arrangements, while functional, proved to have a number of difficulties, such as the matter of alignment to achieve the greatest resolution. Turning to FIG. 4, there is illustrated an exit aperture arrangement in accordance with my invention. A disc, designated by numeral 37 in FIG. 2, provides a greatly simplified and more precise technique of accomplishing the desired improved resolution. The disc 37 is mounted, by means not shown, to be in a fixed position relative to opening 25.

Member 37 is a thin disc of a metal such as molybdenum having an overall thickness of about 0.003 inches. The choice of material for disc 37 is restricted by rigidity requirement for dimensional control in positioning and by the requirement that the disc be nonmagnetic. Copper and gold are satisfactory, although molybdenum has greater strength. An opening 38 is provided at one portion thereof to permit passage of tube 36 therethrough. Central to the disc 37 is a series of arcuate openings 39, which together form the major portion of an annular ring opening. Connecting links of the original metal of member 37, identified 40, are intermediate segments of the arcuate opening to act as supports for the interior disc 41.

As will be readily appreciated from a consideration of the above articles, the structure illustrated in FIG. 4 accomplishes the improved resolution in a greatly simplified and more precise manner than the teachings of prior art references referred to above.

I claim:

1. Apparatus for performing analyses comprising:

a. means defining an evacuated region;

b. an electron analyzer within said region including inner and outer electrically conductive concentric cylinders, the inner of said cylinders having openings in the wall thereof at sapced intervals from each end thereof, said openings being arranged in generally annular configuration and the annular opening at one end thereof being an entry and the annular opening at the other end thereof being an exit for electrons being analyzed;

c. an electron gun mounted within said inner cylinder and positioned intermediate said annular openings in said inner cylinder, said electron gun oriented and arranged to direct a beam of electrons toward a first end of said inner cylinder;

d. support means for positioning a sample to be analyzed near said first end of said inner cylinder and in line with the beam of electrons emitted by said electron gun whereby Auger electrons are emitted from said sample and pass through the entry annular opening in said inner cylinder;

e. means for impressing unidirectional voltage across said cylinders whereby the Auger electrons which have passed through said entry opening are deflected and those of predetermined energy pass through said exit opening; and,

f. detecting means near the opposite end of said inner cylinder for detecting the electrons which pass through the analyzer.

2. Apparatus in accordance with claim 1 wherein said inner and outer cylinders are spaced from one another by an annularly shaped ceramic insulator positioned adjacent each outer end thereof.

3. Apparatus in accordance with claim 2 wherein each of said ceramic insulators has a conductive coating over the interior surface thereof and in electrical contact with said inner and outer cylinders providing a resistance of from about up to about 100 megohms therebetween.

4. Apparatus inaccordance with claim 3 wherein said conductive coating is a cermet of nickel and a ceramic selected from the group consisting of quartz and alumina.

5. Apparatus in accordance with claim 1 wherein the annular openings in said inner cylinder have a metallic mesh thereacross.

6. Apparatus in accordance with claim 1 wherein an off-axis exit aperture is provided within said inner cylinder intermediate said exit annular opening and said detecting means, said aperture being defined by a plurality of arcuate openings ofa generally annular configuration in a thin disc-shaped member positioned perpendicular to the longitudinal axis of said inner cylinder.

7. Apparatus in accordance with claim 6 wherein said exit aperture is formed in a disc-shaped member of about 0.003 inches thickness formed of a metal selected from the group consisting of copper, gold and molybdenum.

8. Apparatus in accordance with claim 2 wherein each of said ceramic insulators has a coating over the interior surface thereof which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said inner and said outer cylinders.

9. Apparatus for performing analyses comprising:

a. an electron analyzer including inner and outer electrically conductive cylinders;

b. the inner of said cylinders having openings in the walls thereof at spaced intervals from each end thereof, said openings being arranged in generally annular configuration;

c. an electron gun mounted within said inner cylinder and positioned intermediate said annular openings in said inner cylinder, said electron gun oriented and arranged to direct a beam of electrons outwardly toward an open end of said inner cylinder and onto a sample to be analyzed; means for impressing unidirectional electrical voltage across said cylinders; and

e. means for detecting the electrons emitted by a sample which pass through the analyzer.

10. Apparatus in accordance with claim 9 wherein said inner and outer cylinders are spaced from one another by an annularly shaped ceramic insulator positioned adjacent each outer end thereof.

11. Apparatus in accordance with claim 10 wherein each of said insulators has a conductive coating over the interior surface thereof and in electrical contact with said inner and outer cylinders providing a resistance of from about 10 up to about megohms therebetween.

12. Apparatus in accordance with claim 10 wherein each of said insulators has a coating over the interior surface thereof which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said inner and outer cylinders.

13. Apparatus in accordance with claim 9 wherein said annular openings in said inner cylinder have a me tallic mesh thereacross.

14. Apparatus in accordance with claim 9 wherein an off-axis exit aperture is provided within said inner cylinder intermediate the exit annular opening in said inner cylinder for electrons being analyzed and said detecting means, said aperture being defined by a plurality of arcuate openings in a generally annular configuration in a thin disc-shaped member positioned perpendicular to the longitudinal axis of said inner cylinder.

15. In an electron analyzer including a source of high energy electrons for irradiating a specimen, inner and outer electrically conductive concentric cylinders, the inner of said cylinders having openings in the walls thereof in generally annular configuration near each end thereof to permit electrons to pass therethrough and be analyzed by the presence of an electric field beend of said inner cylinder, the improvement comprising an exit aperture disc positioned intermediate said detector and the exit opening in the wall of said inner cylinder, said disc being of a thin metallic plate having a plurality of arcuate openings therethrough in a generally annular configuration and spaced from the center axis thereof so as to pass substantially only electrons of a predetermined energy that have been analyzed by said cylinders, said disc mounted within and perpendicular to the longitudinal axis of said inner cylinder.

16. In a cylindrical electron analyzer including a source of high energy of electrons for irradiating a specimen, inner and outer concentric electrically conductive cylinders, the inner of said cylinders having openings in the walls thereof near each end thereof to permit electrons to pass therethrough and be analyzed by the presence of an electric field between said inner and outer cylinders and a detecting means for electrons so analyzed, the improvement comprising spacing said inner and outer cylinders by means of a ceramic insulator at each end thereof, said insulators having a conductive coating on the face thereof interior to said cylinders, and in electrical contact therewith, said conductive coating being formed of a cermet material and providing a resistance of from about 10 up to about megohms between said cylinders whereby a more uniform potential field is formed between said cylinders.

17. In a cylindrical electron analyzer including a source of high energy electrons for irradiating a specimen, inner and outer concentric electrically conductive cylinders, the inner of said cylinders having openings in the walls thereof near each end thereofto permit electrons to pass therethrough and be analyzed by the presence of an electric field between said inner and outer cylinders and a detecting means for electrons so analyzed, the improvement comprising spacing said inner and outer cylinders by means of a ceramic insulator at each end thereof, said insulators having a coating on the face thereof interior to said cylinders which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said cylinders. 

1. Apparatus for performing analyses comprising: a. means defining an evacuated region; b. an electron analyzer within said region including inner and outer electrically conductive concentric cylinders, the inner of said cylinders having openings in the wall thereof at sapced intervals from each end thereof, said openings being arranged in generally annular configuration and the annular opening at one end thereof being an entry and the annular opening at the other end thereof being an exit for electrons being analyzed; c. an electron gun mounted within said inner cylinder and positioned intermediate said annular openings in said inner cylinder, said electron gun oriented and arranged to direct a beam of electrons toward a first end of said inner cylinder; d. support means for positioning a sample to be analyzed near said first end of said inner cylinder and in line with the beam of electrons emitted by said electron gun whereby Auger electrons are emitted from said sample and pass through the entry annular opening in said inner cylinder; e. means for impressing unidirectional voltage across said cylinders whereby the Auger electrons which have passed through said entry opening are deflected and those of predetermined energy pass through said exit opening; and, f. detecting means near the opposite end of said inner cylinder for detecting the electrons which pass through the analyzer.
 2. Apparatus in accordance with claim 1 wherein said inner and outer cylinders are spaced from one another by an annularly shaped ceramic insulator positioned adjacent each outer end thereof.
 3. Apparatus in accordance with claim 2 wherein each of said ceramic insulators has a conductive coating over the interior surface thereof and in electrical contact with said inner and outer cylinders providing a resistance of from about 10 up to about 100 megohms therebetween.
 4. Apparatus in accordance with claim 3 wherein said conductive coating is a cermet of nickel and a ceramic selected from the group consisting of quartz and alumina.
 5. Apparatus in accordance with claim 1 wherein the annular openings in said inner cylinder have a metallic mesh thereacross.
 6. Apparatus in accordance with claim 1 wherein an off-axis exit aperture is provided within said inner cylinder intermediate said exit annular opening and said detecting means, said aperture being defined by a plurality of arcuate openings of a generally annular configuration in a thin disc-shaped member positioned perpendicular to the longitudinal axis of said inner cylinder.
 7. Apparatus in accordance with claim 6 wherein said exit aperture is formed in a disc-shaped member of about 0.003 inches thickness formed of a metal selected from the group consisting of copper, gold and molybdenum.
 8. Apparatus in accordance with claim 2 wherein each of said ceramic insulators has a coating over the interior surface thereof which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said inner and said outer cylinders.
 9. Apparatus for performing analyses comprising: a. an electron analyzer including inner and outer electrically conductive cylinders; b. the inner of said cylinders having openings in the walls thereof at spaced intervals from each end thereof, said openings being arranged in generally annular configuration; c. an electron gun mounted within said inner cylinder and positioned intermediate said annular openings in said inner cylinder, said electron gun oriented and arranged to direct a beam of electrons outwardly toward an open end of said inner cylinder and onto a sample to be analyzed; d. means for impressing unidirectional electrical voltage across said cylinders; and e. means for detecting the electrons emitted by a sample which pass through the analyzer.
 10. Apparatus in accordance with claim 9 wherein said inner and outer cylinders are spaced from one another by an annularly shaped ceramic insulator pOsitioned adjacent each outer end thereof.
 11. Apparatus in accordance with claim 10 wherein each of said insulators has a conductive coating over the interior surface thereof and in electrical contact with said inner and outer cylinders providing a resistance of from about 10 up to about 100 megohms therebetween.
 12. Apparatus in accordance with claim 10 wherein each of said insulators has a coating over the interior surface thereof which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said inner and outer cylinders.
 13. Apparatus in accordance with claim 9 wherein said annular openings in said inner cylinder have a metallic mesh thereacross.
 14. Apparatus in accordance with claim 9 wherein an off-axis exit aperture is provided within said inner cylinder intermediate the exit annular opening in said inner cylinder for electrons being analyzed and said detecting means, said aperture being defined by a plurality of arcuate openings in a generally annular configuration in a thin disc-shaped member positioned perpendicular to the longitudinal axis of said inner cylinder.
 15. In an electron analyzer including a source of high energy electrons for irradiating a specimen, inner and outer electrically conductive concentric cylinders, the inner of said cylinders having openings in the walls thereof in generally annular configuration near each end thereof to permit electrons to pass therethrough and be analyzed by the presence of an electric field between said inner and outer cylinders and wherein a detector of said analyzed electrons is provided at the exit end of said inner cylinder, the improvement comprising an exit aperture disc positioned intermediate said detector and the exit opening in the wall of said inner cylinder, said disc being of a thin metallic plate having a plurality of arcuate openings therethrough in a generally annular configuration and spaced from the center axis thereof so as to pass substantially only electrons of a predetermined energy that have been analyzed by said cylinders, said disc mounted within and perpendicular to the longitudinal axis of said inner cylinder.
 16. In a cylindrical electron analyzer including a source of high energy of electrons for irradiating a specimen, inner and outer concentric electrically conductive cylinders, the inner of said cylinders having openings in the walls thereof near each end thereof to permit electrons to pass therethrough and be analyzed by the presence of an electric field between said inner and outer cylinders and a detecting means for electrons so analyzed, the improvement comprising spacing said inner and outer cylinders by means of a ceramic insulator at each end thereof, said insulators having a conductive coating on the face thereof interior to said cylinders, and in electrical contact therewith, said conductive coating being formed of a cermet material and providing a resistance of from about 10 up to about 100 megohms between said cylinders whereby a more uniform potential field is formed between said cylinders.
 17. In a cylindrical electron analyzer including a source of high energy electrons for irradiating a specimen, inner and outer concentric electrically conductive cylinders, the inner of said cylinders having openings in the walls thereof near each end thereof to permit electrons to pass therethrough and be analyzed by the presence of an electric field between said inner and outer cylinders and a detecting means for electrons so analyzed, the improvement comprising spacing said inner and outer cylinders by means of a ceramic insulator at each end thereof, said insulators having a coating on the face thereof interior to said cylinders which is electrically conductive relative to said insulator and highly resistive relative to said cylinders, said coating being in electrical contact with said cylinders. 